SYN-SEISMIC DEPOSITION OR SYNDEPOSITIONAL SEISMICITY: PALEOSEISMITES RECORD 10 M.Y. OF EARLY PALEOGENE LARAMIDE SEISMICITY IN SAN JUAN BASIN, NORTH-CENTRAL NEW MEXICO

Features interpreted as paleoseismites in eastern San Juan Basin are observed in Paleocene–Eocene fluvial siliciclastic sedimentary rocks proximal to the basin-bounding Nacimiento fault. Features include clastic dikes, convolute bedding, diapir-like structures, vents, anastomosing vein-like structures, and potential thixotropic bowls the early Paleocene (ca. 66.0 Ma) Ojo Alamo Sandstone, early to middle Paleocene (ca. 65.8 – 62.3 Ma) Nacimiento Formation, and early Eocene (ca. 56.0 Ma) Cuba Mesa Member of San Jose Formation. Observations to date suggest that these features are limited to areas within 20 km of the Nacimiento fault, and have not been found farther north where the Nacimiento fault terminates northward into the Archuleta anticlinorium or to the west further into the basin. Convolute bedding is often truncated by overlying beds, indicating soft-sediment deformation at or near the surface before deposition of overlying strata. Clastic dike measurements thus far reveal two major strike orientations: 055° (dominant) and 160° (subsidiary). The dominant clastic dike orientation is sub-parallel to prior estimates of Cretaceous–Paleogene WSW-ENE intraforeland (i.e., Laramide) shortening at the local to regional scale. Cross-cutting planar features that strike 020° may post-date the clastic dikes, but their relationship is not yet clear. The presence of paleoseismites in earliest Paleocene (Puercan North American Land Mammal Age) through early Eocene (Clarkforkian–Wasatchian age) indicates that seismicity occurred for at least approximately 10 million years in the early Paleogene San Juan Basin, consistent with stratigraphic evidence of tectonism at this time. The presence of San Juan Basin paleoseismites could suggest early Cenozoic intraforeland earthquakes on the order M ≥ 5 that disrupted sedimentation patterns prior to lithification. These preliminary data suggest a 10 m.y. episode of intermittent seismicity associated with the Cenozoic structural and depositional evolution of the San Juan Basin during the Laramide orogeny. Results from this work will be integrated into regional studies on the paleoenvironmental, depositional, and Laramide tectonic development of the area.